1. Field of the Invention
The invention relates to an apparatus and method for depositing multilayer thin films in a magnetron sputtering process. More particularly, the invention relates to an apparatus and method for depositing thin magnetic films for magnetic recording media in a high volume, electronically controlled, magnetron sputtering process, and to production of an improved magnetic recording disk product thereby.
2. Description of the Related Art
Sputtering is a well-known technique for depositing uniform thin films on a particular substrate. Sputtering is performed in an evacuated chamber using an inert gas, typically argon, with one or more substrates remaining static during deposition, being rotated about the target (a "planetary" system) or being transported past the target (an "in-line" system).
Fundamentally, the technique involves bombarding the surface of a target material to be deposited as the film with electrostatically accelerated argon ions. Generally, electric fields are used to accelerate ions in the plasma gas, causing them to impinge on the target surface. As a result of momentum transfer, atoms and electrons are dislodged from the target surface in an area known as the erosion region. Target atoms deposit on the substrate, forming a film.
Typically, evacuation of the sputtering chamber is a two-stage process in order to avoid contaminant-circulating turbulence in the chamber. First, a throttled roughing stage slowly pumps down the chamber to a first pressure, such as about 50 microns. Then, high vacuum pumping occurs using turbo-, cryo- or diffusion pumps to evacuate the chamber to the highly evacuated base pressure (about 10.sup.-7 Torr) necessary to perform sputtering. Sputtering gas is subsequently provided in the evacuated chamber, backfilling to a pressure of about 2-10 microns.
The sputtering process is useful for depositing coatings from a plurality of target materials onto a variety of substrate materials, including glass, nickel-phosphorus plated aluminum disks, and ceramic materials. However, the relatively low sputtering rate achieved by the process solely relying on electrostatic forces (diode sputtering) may be impracticable for certain commercial applications where high volume processing is desired. Consequently, various magnet arrangements have been used to enhance the sputtering rate by trapping electrons close to the target surface, ionizing more argon, increasing the probability of impacting and dislodging target atoms and therefore the sputtering rate. In particular, an increased sputtering rate is achieved by manipulation of a magnetic field geometry in the region adjacent to the target surface.
Sputter deposition performed in this manner is generally referred to as magnetron sputtering.
The magnetic field geometry may be optimized by adjusting the polarity and position of individual magnets used to generate magnetic fields with the goal of using the magnetic field flux paths to enhance the sputtering rate. For example, U.S. Pat. No. 4,166,018, issued Aug. 28, 1989 to J. S. Chapin and assigned to Airco, Inc., describes a planar direct current (d.c.) magnetron sputtering apparatus which uses a magnet configuration to generate arcuate magnetic flux paths (or lines) that confine she electrons and ions in a plasma region immediately adjacent to the target erosion region. A variety of magnet arrangements are suitable for this purpose, as long as one or more closed loop paths of magnetic flux is parallel to the cathode surface, e.g., concentric ovals or circles.
The role of the magnetic field is to trap moving electrons near the target. The field generates a force on the electrons, inducing the electrons to take a spiral path about the magnetic field lines. Such a spiral path is longer than a path along the field lines, thereby increasing the chance of the electron ionizing a plasma gas atom, typically argon. In addition, field lines also reduce electron repulsion away from a negatively biased target. As a result, a greater ion flux is created in the plasma region adjacent to the target with a correspondingly enhanced erosion of target atoms from an area which conforms to a shape approximating the inverse shape of the field lines. Thus, if the field above the target is configured in arcuate lines, the erosion region adjacent to the field lines conforms to a shallow track, leaving much of the target unavailable for sputtering.
Even lower target utilization is problematic for magnetic targets because magnetic field lines tend to be concentrated within, and just above, a magnetic target. With increasing erosion of the magnetic target during sputtering, the field strength above the erosion region increases as more field lines `leak` out from the target, trapping more electrons and further intensifying the plasma close to the erosion region. Consequently, the erosion region is limited to a narrow valley.
In addition to achieving high film deposition rates, sputtering offers the ability to tailor film properties to a considerable extent by making minor adjustments to process parameters. Of particular interest are processes yielding films with specific crystalline microstructures and magnetic properties. Consequently, much research has been conducted on the effects of sputtering pressures, deposition temperature and maintenance of the evacuated environment to avoid contamination or degradation of the substrate surface before film deposition.
Alloys of cobalt, nickel and chromium deposited on a chromium underlayer (CoNiCr/Cr) are highly desirable as films for magnetic recording media such as disks utilized in Winchester-type hard disk drives. However, on disk substrates, in-line sputtering processes create magnetic anisotropies which are manifested as signal waveform modulations and anomalies in the deposited films.
Anisotropy in the direction of disk travel through such in-line processes is understood to be caused by crystalline growth perpendicular to the direction of disk travel as a result of the deposition of the obliquely incident flux of target atoms as the disk enters and exits a sputtering chamber. Since magnetic film properties depend on crystalline microstructure, such anisotropy in the chromium underlayer can disrupt the subsequent deposition of the magnetic CoNiCr layer in the preferred orientation. The preferred crystalline orientation for the chromium underlayer is with the closely packed, bcc {110} plane parallel to the film surface. This orientation for the chromium nucleating layer forces the `C` axis of the hcp structure of the magnetic cobalt-alloy layer, i.e., the easy axis of magnetization, to be aligned in the film plane. Similarly, the orientation of the magnetic field generated in the sputtering process may induce an additional anisotropy which causes similar signal waveform modulations. See, Uchinami, et al., "Magnetic Anisotropies in Sputtered Thin Film Disks", IEEE Trans. Magn., Vol. MAG-23, No. 5, 3408-10, September 1987, and Hill, et al., "Effects of Process Parameters on Low Frequency Modulation on Sputtered Disks for Longitudinal Recording", J. Vac Sci. Tech., Vol. A4, No. 3, 547-9, May 1986 (describing magnetic anisotropy phenomena).
Several approaches have been taken to eliminate the aforementioned waveform modulation problems while enhancing magnetic properties in the coating, especially coercivity. For instance, U.S. Pat. No. 4,816,127, issued Mar. 28, 1989 to A. Eltoukhy and assigned to Xidex Corp., describes one means for shielding the substrate to intercept the obliquely incident target atoms. In addition, Teng, et al., "Anisotropy-Induced Signal Waveform Modulation of DC Magnetron Sputtered Thin Films", IEEE Trans. Magn., Vol. MAG-22, 579-581, 1986, and Simpson, et al., "Effect of Circumferential Texture on the Properties of Thin Film Rigid Recording Disks". IEEE Trans. Magn., Vol. MAG-23, No. 5, 3405-7, September 1987, suggest texturizing the disk substrate surface prior to film deposition. In particular, the authors propose circumferential surface grooves to promote circumferentially oriented grain growth and thereby increase film coercivity.
Other approaches to tailoring film properties have focused on manipulating the crystalline microstructure by introducing other elements into the alloy composition. For example, Shiroishi, et al., "Read and Write Characteristics of Co-Alloy/Cr Thin Films for Longitudinal Recording", IEEE Trans. Magn., Vol. MAG-24, 2730-2, 1988, and U.S. Pat. No. 4,652,499, issued Mar. 24, 1987 to J. K. Howard and assigned to IBM, relate to the substitution of elements such as platinum (Pt), tantalum (Ta), and zirconium (Zr) into cobalt-chromium (CoCr) films to produce higher coercivity and higher corrosion resistance in magnetic recording films.
CoCr alloys with tantalum (CoCrTa) are particularly attractive films for magnetic recording media. For example, it is known in the prior art to produce CoCrTa films by planetary magnetron sputtering processes using individual cobalt, chromium and tantalum targets or using cobalt-chromium and tantalum targets.
Fisher, et al., "Magnetic Properties and Longitudinal Recording Performance of Corrosion Resistant Alloy Films", IEEE Trans. Magn., Vol. MAG 22, no. 5, 352-4, September 1986, describe a study of the magnetic and corrosion resistance properties of sputtered CoCr alloy films. Substitution of 2 atomic percent (at.%) Ta for Cr in a Co-16 at.% Cr alloy (i.e., creating a Co-14 at.% Cr-2 at.% Ta alloy) was found to improve coercivity without increasing the saturation magnetization. In particular, a coercivity of 1400 Oe was induced in a 400 .ANG. film. In addition, linear bit densities from 8386 flux reversals/cm to 1063 flux reversals/cm (21300 fci to 28100 fci) were achieved at -3 dB, with a signal-to-noise (SNR) ratio of 30 dB. Moreover, corrosion resistance of CoCr and CoCrTa films was improved relative to CoNi films.
U.S. Pat. No. 4,940,548, issued on Aug. 21, 1990 to Furusawa, et al., and assigned to Hitachi, Ltd., discloses the use of Ta to increase the coercivity and corrosion resistance of CoCr (and CoNi) alloys. CoCr alloys with 10 at.% Ta (and chromium content between 5 and 25 at.%) were sputtered onto multiple layers of chromium to produce magnetic films with low modulation even without texturing the substrate surface and highly desirable crystalline microstructure and magnetic anisotropy.
Development of a high throughput in-line system to produce sputtered CoCrTa films with enhanced magnetic and corrosion-resistance properties for the magnetic recording media industry has obvious economic advantages.
Linear recording density of magnetic films on media used in Winchester-type hard disk drives is known to be enhanced by decreasing the flying height of the magnetic recording head above the recording medium. With reduced flying height, there is an increased need to protect the magnetic film layer from wear. Magnetic films are also susceptible to corrosion from vapors present even at trace concentrations within the magnetic recording system. A variety of films have been employed as protective overlayers for magnetic films, including rhodium, carbon and inorganic nonmetallic carbides, nitrides and oxides, like silica or alumina. However, problems such as poor adhesion to the magnetic layer and inadequate wear resistance have limited the applicability of these films. U.S. Pat. No. 4,503,125 issued on Mar. 3, 1985 to Nelson, et al. and assigned to Xebec, Inc. describes a protective carbon overcoating for magnetic films where adhesion is enhanced by chemically bonding a sputtered layer of titanium between the magnetic layer and the carbon overcoating.
In the particular case of sputtered carbon, desirable film properties have been achieved by carefully controlling deposition parameters. For example, during the sputtering process, the amount of gas incorporated in the growing film depends on sputtering parameters like target composition, sputtering gas pressure and chamber geometry. U.S. Pat. No. 4,839,244, issued on Jun. 13, 1989 to Y. Tsukamoto and assigned to NEC Corp., describes a process for co-sputtering a protective graphite fluoride overlayer with an inorganic nonmetallic compound in a gaseous atmosphere which includes fluorine gas. U.S. Pat. No. 4,891,114 issued on Jan. 1, 1990 to Hitzfeld, et al., and assigned to BASF Aktiengesellschaft of Germany, relates to a d. c. magnetron sputtering process for an amorphous carbon protective layer using a graphitic carbon target.
As the wear-resistant layer for magnetic recording media, it is desirable that the carbon overlayer have a microcrystalline structure corresponding to high hardness. In other words, it is desirable during sputtering to minimize graphitization of carbon which softens amorphous carbon films. One means employed to moderate graphitization of sputtered carbon films is by incorporating hydrogen into the film. Such incorporation may be accomplished by sputtering in an argon atmosphere mixed with hydrogen or a hydrogen-containing gas, such as methane or other hydrocarbons.
Magnetron sputtering processes have been developed which have been somewhat successful in achieving high throughput. For example, U.S. Pat. Nos. 4,735,840 and 4,894,133 issued to Hedgcoth on Apr. 5, 1988 and Apr. 16, 1990, respectively, describe a high volume planar d. c. magnetron in-line sputtering apparatus which forms multilayer magnetic recording films on disks for use in Winchester-type hard disk technology. The apparatus includes several consecutive regions for sputtering individual layers within a single sputtering chamber through which preheated disk substrates mounted on a pallet or other vertical carrier proceed at velocities up to about 10 mm/sec (1.97 ft/min), though averaging only about 3 mm/sec (0.6 ft/min). The first sputtering region deposits chromium (1,000 to 5,000 .ANG.) on a circumferentially textured disk substrate. The next region deposits a layer (200 to 1,500 .ANG.) of a magnetic alloy such as CoNi. Finally, a protective layer (200 to 800 .ANG.) of a wear- and corrosion-resistant material such as amorphous carbon is deposited.
The apparatus is evacuated by mechanical and cryo pumps to a base pressure about 2.times.10.sup.-7 Torr. Sputtering is performed at a relatively high argon pressure between 2 and 4.times.10.sup.-2 Torr (20 to 40 microns) to eliminate anisotropy due to obliquely incident flux.
In optimizing a sputtering process to achieve high throughput, consideration should be given to other time-influenced aspects of the process apart from the sputtering rate. For example, substrate heating is typically accomplished with heaters requiring an extended dwell time to warm substrates to a desired equilibrium temperature. In addition, substrate transport speeds through the sputtering apparatus have been limited with respect to mechanisms using traditional bottom drive, gear/belt-driven transport systems. Such bottom drive systems generally have intermeshing gears and may be practically incapable of proceeding faster than a particular rate due to rough section-to-section transitions which may dislodge substrates from the carrier and/or create particulate matter from gear wear which contaminates the disks prior to or during the sputtering process. Thus, overall process throughput would be further enhanced by the employment of heating and transport elements which require minimal time to perform these functions.
Generally, prior art sputtering devices utilize relatively unsophisticated means for controlling the sputtering processes described therein. Such control systems may comprise standard optical or electrical metering monitored by a system operator, with direct electrical or electro-mechanical switching of components utilized in the system by the system operator. Such systems have been adequately successful for limited throughput of sputtered substrates. However, a more comprehensive system is required for higher throughput sputtering operations. Specifically, a control system is required which provides to the operator an extensive amount of information concerning the ongoing process through a relatively user-friendly environment. In addition, the control system must adequately automate functions both in series and in parallel where necessary to control every aspect of the sputtering system. Further, it is desirable to include within such a control system the capability to preset a whole series of operating parameters to facilitate rapid set-up of the system for processes employing myriad sputtering conditions.